This invention relates to a transparent coated composite which is formed by applying a precoating and then an over-coating on a substrate mainly comprising polymethyl methacrylate, said composite having excellent surface abrasion resistance and extremely good adhesion or bonding between the coated films and the substrate.
Among the materials known as transparent organic glass, presently the most commonly used material is polymethyl methacrylate. This resin is excellent in transparency, weatherability, etc., but is dissatisfactory in that the surface of the products of this resin is easily scratched. Improvement in abrasion resistance of the resin is therefore desired in various fields of use thereof. In this regard, many investigations have been made by now, but all of those have both merits and demerits and do not give the resin practically satisfactory properties.
For example, Japanese Patent Publication No. 17847/60 discloses a process for improving surface hardness of polymethyl methacrylate by applying a cross-linking monomer such as diethylene or triethylene glycol dimethacrylate, a C.sub.3 -C.sub.6 alkane diol dimethacrylate, etc. filmily inside a mold, followed by cast polymerization of methyl methacrylate therein. Japanese Patent Publication No. 9827/62 discloses a process for improving surface hardness of polymers such as polymethyl methacrylate by applying a mixture of a cross-linking monomer such as allyl methacrylate, ethylene dimethacrylate, etc. and a polymerization catalyst inside a mold, then placing a polymer body previously made by said mold in contact with the applied cross-linking monomer coating and heating the system. Japanese Laying-Open Publication No. 26419/72 discloses a process in which a solution of a hydrolysate of an alkyl silicate such as butyl silicate in admixture with, e.g., a copolymer of vinyltriethoxysilane and vinyl acetate in a solvent is applied on a polymethyl methacrylate plate, which is then heated so as to cure the coating.
However, the processes described in Japanese Patent Publication Nos. 17847/60 and 9827/62 have defects in that prolonged time is required for curing and the surface of the products is insufficient in abrasion resistance. Also the process described in Japanese Laying-Open Publication No. 26419/72 has a fatal defect in heat resistance of the product. That is, when heated at 80.degree. C. or higher for a few minutes, the cured coating on the surface of polymethyl methacrylate resin cracks and loses its optical characteristics.
For the purpose of overcoming these defects we made many studies and previously found that a silicon compound of the undermentioned Group II or a mixture of the silicon compound of Group II and a polymerizable monomer of the undermentioned Group III (or its polymer) has high abrasion resistance.
Said silicon compound or the mixture thereof with said polymerizable monomer can be coated on a transparent plastic substrate and cured to successfully impart excellent abrasion resistance to the substrate. However, the cured coating formed on a substrate mainly comprising polymethyl methacrylate, which provides the substrate with abrasion resistance, is not sufficiently bonded to the resin substrate. Therefore, further measures to strengthen the adhesion or bonding between the coating and the substrate had to be searched for. In this regard, we tried various measures for the improvement and have accomplished this invention.